1. The Field of the Invention
The present invention relates generally to the desulfurization of crude oil and other hydrocarbons, more particularly hydrodesulfurization. In particular, the present invention relates to the use of an oil-soluble, bifunctional catalyst that is used in a hydrodesulfurization process to selectively remove sulfur.
2. Related Technology
World demand for refined fossil fuels is ever-increasing and will eventually outstrip the supply of high quality crude oil. As the shortage of high quality crude oil increases, there is an increasing demand to find better ways to exploit lower quality feedstocks and extract fuel values from them.
Lower quality feedstocks are characterized by relatively high quantities of hydrocarbons that have a boiling point of 524° C. (975° F.) or higher. They also typically contain relatively high concentrations of sulfur, nitrogen and/or metals. High boiling fractions typically have a high molecular weight and/or low hydrogen/carbon ratio, an example of which is a class of complex compounds collectively referred to as “asphaltenes”. Asphaltenes are difficult to process and commonly cause fouling of conventional catalysts and hydroprocessing equipment.
Examples of lower quality feedstocks that contain relatively high concentrations of asphaltenes, sulfur, nitrogen and metals include sour crude, heavy crude, sour-heavy crude, oil sands bitumen, and bottom of the barrel and residuum left over from conventional refinery processes (collectively “heavy oil”). The terms “bottom of the barrel” and “residuum” (or “resid”) typically refer to atmospheric tower bottoms, which have a boiling point of at least 343° C. (650° F.), or vacuum tower bottoms, which have a boiling point of at least 524° C. (975° F.). The terms “resid pitch” and “vacuum residue” are commonly used to refer to fractions that have a boiling point of 524° C. (975° F.) or greater.
Despite the industry's extensive research there are certain crude oils, distillates, and/or by-products of the various refining processes that are difficult to upgrade. Converting heavy oil into useful end products requires extensive processing, including reducing the boiling point of the heavy oil, increasing the hydrogen-to-carbon ratio, and removing impurities such as metals, sulfur, nitrogen and carbon forming compounds. Hydrocarbons containing these types of hydrocarbons tend to form coke under many refining conditions, which can foul reactors and reduce the yield of useful or high-value petroleum products. Consequently, these products are often used “as is” in their low-value form.
There is a persistent desire in the industry to convert these low-value feedstocks into high-value petroleum. For example, the price premium paid for a barrel of sweet-light crude oil versus a barrel of sour-heavy crude oil ranges from about $5-20, depending on the overall price of oil.
However, even with the large price premium paid for high-quality petroleum, it is often not economical to desulfurize low-value petroleum by hydrodesulfurization. This is due at least in part to low desulfurization rates coupled with high consumption of hydrogen. Moreover, the catalysts used in hydrodesulfurization processes become fouled or rapidly undergo catalyst deactivation. The undesirable reactions and fouling involved in hydrodesulfurization of heavy oil greatly increases the materials and maintenance costs of processing heavy oils.
Even small improvements in catalyst performance can have a significant benefit to the cost of the hydrodesulfurization process due to the increase in output and/or the reduced use of the catalyst. Despite the continuing pressures to catalytically upgrade heavy hydrocarbons or low-value feedstocks, there is a long-felt but unsatisfied need in the industry for processes and catalysts that increase the efficiency and economy of upgrading low-value petroleum materials by hydrodesulfurization.